All linear polyesters known for food packaging are, at the present time, primarily produced in such a manner that a precondensate, obtained by direct esterification or by ester interchange, can only be brought to a medium level of viscosity by melt polycondensation. Higher viscosities are attained by means of a subsequent solid phase polycondensation, which generally is preceded by pelletizing and crystallization.
Polyesters for food packaging must have, apart from a high viscosity, the lowest possible concentration of free acetaldehyde and acetaldehyde which is bound as vinyl ester, since the acetaldehyde gives packaged foods an unpleasant taste and smell. In the case of mineral waters, less than 3 ppm acetaldehyde, corresponding to less than 1 ppm of free acetaldehyde and less than 10 ppm of bound acetaldehyde in the pellets used for the production of the bottle, should be detectable within the wall of the bottle. Still lower values would be preferable. Non-colored packaging should be highly transparent and subjectively colorless,